1. Field of the Invention
This invention relates to gas-liquid contacting devices and the use of such devices in liquid treatment. The invention especially relates to methods and apparatuses for aeration pumping in activated sludge processes, particularly when conducted in oxidation ditches of racetrack or loop channel configuration.
2. Review of the Prior Art
Many liquid waste treatment processes, commonly termed aerobic processes, supply bacteria and other microorganisms with dissolved oxygen for treating aqueous wastes such as municipal sewage, tannery wastes, dairy wastes, meat-processing wastes, and the like.
One such aerobic process is the activated sludge process, in which the microorganisms are concentrated as an activated sludge to be mixed with incoming wastewater, which supplies food for the organisms. The apparatuses in which the activated sludge process is conducted comprise an aeration basin (reactor basin) and a final clarifier (settling tank). The aeration basin serves as a culturing basin in which to generate the growth of bacteria, protozoa, and other types of microorganisms, so that they can consume the pollutants in the raw waste entering the basin by converting the pollutants into energy, carbon dioxide, water, and cells (biomass).
The activated sludge process is effective for controlling this conversion activity within the aeration basin, for settling the biomass within the clarifier, for overflowing the purified liquor of effluent from the clarifier to discharge, and for returning the settled biomass from the clarifier to the aeration basin. Thus the activated sludge process is a suspended-growth, aerobic, biological treatment process, using an aeration basin and a settling tank, which is capable of producing very pure, high quality effluent, as long as the biomass settles properly.
It can thus be compared to a fixed-growth process wherein the growth of the biomass occurs on or within a tower on plastic media or in a trickling filter on rocks packed therewithin.
The activated sludge process is represented by two prime mixing regimes, plug flow and complete mixing, which represent the opposite extremes of a continuum and almost infinite variety of intermediate mixing modes.
Plug-flow is characterized by use of relatively long, narrow aeration tanks or basins into which wastewater, with or without return sludge, is added at one end and from which it flows at the opposite end to enter a clarifier. The inflowing wastewater progressively moves down the tank length, essentially unmixed with the balance of the tank contents. Dissolved oxygen is generally added along the entire length of the basin. Intermediate mixing modes are sometimes termed semi-plug flow systems and include introduction of return sludge and/or wastewater at a plurality of positions along the length of the basin. A disadvantage is that plug-flow systems are inherently dominated by the inflowing wastewater which volumetrically overpowers the returning activated sludge so that temporary or cyclic variations in wastewater characteristics, such as unusually large quantities of materials poisonous to microorganisms, can cause shock loadings that can at least temporarily inactivate the system.
Plug-flow systems are characterized by a dissolved-oxygen gradient. The dissolved-oxygen content is low at the entrance to the elongated basin, where raw waste and activated sludge are generally combined, and increases to a high level at the discharge end of the basin where the pollutants have been substantially consumed. However, plug-flow systems are not operated to include an anoxic zone within the basin.
In addition to its oxygen gradient, a plug-flow system is also characterized by a gradient in oxygen uptake rate of its mixed liquor. The rate is necessarily highest at the inlet end of the plug-flow aeration basin, lowest at the outlet end, and progressively decreasing along its length because the food supply steadily decreases from the inlet end to the outlet end.
Complete-mix systems are designed so that if samples are taken simultaneously over the basin area, the measured properties are essentially uniform as a theoretical aim. As one of these properties, the dissolved-oxygen content (D.O) is maintained as uniformly as possible at an average dissolved-oxygen content of 2.0 mg of O.sub.2 /l. In practice, the D.O. concentration is usually not uniform because higher D.O. concentrations are found closer to the aerators and to the liquid surface (particularly if surface aerators are used) and because lower D.O. concentrations are found near the sides and the bottom of the basin.
Complete mixing is commonly conducted in round or square tanks into which incoming wastes are fed at numerous places. The contents of the tanks are sufficiently mixed to insure that the incoming wastes are rapidly dispersed throughout the tank, in contrast to plug-flow systems. The volume of mixed liquor in the tank is so much greater than the volume of the wastewater that the wastewater is overwhelmingly dominated by the tank contents. Thus there is a relatively uniform food/microorganism ratio existing in such complete-mix tanks. Also, there is a uniform concentration of mixed liquor-suspended solids (MLSS) to be found in complete-mix aeration tanks as contrasted with the variable concentration noted in the plug-flow and semi-plug flow tanks.
An endless fill-and-draw system, using multiple baffles and air diffusers for propulsion and BOD removal in an activated sludge process, is described in U.S. Pat. No. 1,247,542.
As a variation of the activated sludge process, A. Pasveer of the Netherlands received Dutch Pat. No. 87,500 in 1951 for an aeration basin provided with a horizontally mounted rotor having brush surfaces for adding oxygen to sewage and impelling the surface of the sewage to flow in a closed-loop circuit within an ovally laid-out ditch having a racetrack shape in plan view. The ditch was intermittently operated; mixed liquor was circulated and aerated for a period of time, the liquor was then clarified by settling, excess sludge was removed, and wastewater addition and operation of the rotor were resumed. This invention, known as an oxidation ditch, is also disclosed in British Pat. No. 796,438.
In subsequent developments, the intermittently operated oxidation ditch became a continuous system by combining the ditch with a final clarifier so that the oxidation ditch itself became an activated-sludge aeration basin. In addition, the brush rotors were replaced with cage rotors having paddles or blades for chopping into the surface of the water and hurling a portion downstream to create surface aeration and induce the flow of the mixed liquor therebeneath.
A rotor equipped with blades mounted in a ditch having a depth greater than seven feet is illustrated in FIG. 1. Because of this depth, an inclined baffle is positioned about 4 to 15 feet downstream of the rotor in order to provide mixing of aerated liquor near the surface with unaerated liquor which is flowing near the bottom. The stratification that results from operating a ditch without baffles is shown in FIG. 2 as a cross section of a ditch equipped with six horizontal-shaft rotors for treating municipal sewage, rotors 2, 4 and 6 being idle. The hatched zones have a D.O. content of 0.5-1.5 mg O.sub.2 /l, and the unmarked liquor therebeneath has a D.O. content of less than 0.5 mg O.sub.2 /l, according to an article, published in 1976, entitled "Activated Sludge Process II--Nitrogen Removal, Phosphorus Removal, Aeration-Transfer of Pure Oxygen", by Wilhelm von der Emde, Institute fur Wasserversorgung, Abwasser-reinigung and Gewasserschutz, TC Wein, A-1040 Wein, Austria.
In order to provide a system capable of treating high peak flow of wastewater and even excessive storm water flows, an oxidation ditch has been developed which has a channel of varied cross section and is aerated by a horizontal-shaft surface aerator supported on floats. This aerator is depicted in FIG. 3 and is described in U.S. Pat. No. 3,759,495. It is equipped with curved blades and a baffle which prevents the recirculation of freshly aerated fluid immediately back through the device a second time, the aerated fluid being lifted and revolved toward the baffle and then routed around either side of the device.
In another development, cage rotors have been replaced with surface aerators in the form of rotors having horizontally disposed shafts and large-diameter plastic discs mounted transversely thereupon. About forty percent of the surface area of these discs is immersed in the liquor. They have many holes therethrough and operate by rotationally dipping into the surface of the liquor to pump the liquor by hydraulic friction, to bring air therebeneath, and to lift liquor thereabove so that the covering layer of aerobic bio-mass absorbs oxygen and removes organic materials from the wastewater. FIG. 4 is an end view of a horizontal shaft disc aerator operating in an aeration channel.
A further improvement in oxidation ditch systems was described by 1970 in U.S. Pat. No. 3,510,110, comprising the location of a slow-speed mechanical surface aerator, having a vertically disposed shaft, at one end of a longitudinal partition that forms the straight channels of an oxidation ditch, the aerator being disposed close enough to the end of the partition and being so aligned therewith that the partition closes off the circuit on one side of the surface aerator. By providing a highly aerated surface condition and by impacting the circularly toroidal flow upon the longitudinal partition, the flow is converted into a slow spiraling flow downstream of the aerator.
FIG. 5 is a plan view and FIG. 6 is a sectional side view of such an oxidation ditch in which a surface aerator, mounted vertically and close to the dividing wall, creates a complete-mix aeration zone throughout the end of the ditch surrounding the aerator, transfers dissolved oxygen to the mixed liquor, and imparts sufficient velocity to suspend 4,000 mg/l of solids.
Another development that has been principally used in very deep oxidation ditches is the directional mix jet aerator system (eddy jet) which utilizes a plurality of subsurface ejector aerators which are connected to a transversely disposed header at the bottom of the channel as described in U.S. Pat. No. 3,846,292. This system is shown in FIG. 7 as a circular open-channel oxidation ditch having four headers which are connected to a blower and a submersible pump. The mixing pattern is shown as a section through a header and the surrounding mixed liquor in FIG. 8.
U.S. Pat. No. 3,900,394 describes a circuit-flow oxidation ditch having a vertically mounted, impeller-type mechanical surface aerator at one or both ends which emphasizes the use of an oxidation ditch for denitrification in an activated sludge extended aeration process. At a loading of 6000-8000 mg/l of mixed liquor suspended solids and at a depth up to 14 feet, this system is described as capable of maintaining suspension of the solids throughout a channel length of up to 900 feet.
It is pertinent to note that a conventional circuit-flow oxidation ditch of the prior art operates as a complete-mix system except that its D.O. gradient is characteristically plug-flow. Circulation of the entire basin contents during each cycle, while admixing the mixed liquor with the relatively minor stream of inflowing wastewater, ensures such complete-mix conditions.
Although it could be stretched out so that its racetrack or looped channel would be a mile in length, for example, so that the circuit flow in its channel would be comparable to that of the inflowing wastewater in volume, such as 1:1 to 3:1 (the latter being a dilution ratio for settled sewage in U.S. Pat. No. 1,643,273 of Imhoff, for example), thereby simulating a true plug-flow activated-sludge system, it would then be subject to shock-load effects, the food-to-microorganism ratio being so high that the microorganisms could readily be overwhelmed by incoming poisons or other changes in the food situation. Preferably, therefore, an oxidation ditch is sufficiently short that its channel flow of mixed liquor is ample to dilute the inflowing wastewater by volume ratios of 100:1 to 200:1 or greater, whereby the inflowing wastewater is completely dominated volumetrically by the mixed liquor in the ditch and the food-to-microorganism ratio is low enough that the microorganisms can handle any reasonable change in food properties, thereby simulating a true complete-mix system.
At such desirable volume ratios, an oxidation ditch can be designed to operate with recycled sludge within its channel on a food-to-microorganism ratio (F/M) by weight that varies over a possible range of 0.01 to 5.0, depending upon space, cost, and process design requirements, by varying the concentration of microorganisms, expressed as mixed liquor suspended solids (MLSS), flowing within its channel. If operating at a low F/M ratio of 0.01-0.2, it is an extended aeration system, producing small quantities of sludge. If operating at a medium F/M ratio of 0.2-0.5, it is a conventional system. If operating at a high F/M ratio of 0.5-2.5, it is a high-rate activated sludge system, producing large quantities of sludge. Moreover, it can even be operated as an aerated lagoon with no recycle sludge at F/M ratios above 2.5, but it is then not operating according to the activated sludge process and is therefore not herein defined as an oxidation ditch.
An oxidation ditch may also shift through a wide F/M range, representing all three of these systems, as it begins operation as a high-rate activated sludge system, with no built-up sludge, and gradually builds up its recycled sludge to a mixed liquor suspended solids (MLSS) content of 3,000 mg/l where extended aeration can generally be considered to begin. In general, an oxidation ditch is considered for design purposes to exist when the MLSS content reaches about 1500 mg/l, because at lower levels the size of the ditch would have to be excessive, but the principles of its operation are nevertheless applicable at much lower MLSS levels, such as at 1,000 mg/l.
It is significant that increasing the concentration of the microorganisms increases the total amount of oxygen used in an oxidation ditch of given volume and necessitates a higher flow velocity to maintain the greater mass of solids in suspension. At a given rate of food inflow (F), increasing the concentration (M) of microorganisms obviously decreases the F/M ratio. A change in the F/M ratio also affects the O.sub.2 transfer rate (measured as pounds of oxygen per hour at process conditions) for which the ditch must be designed, as is known in the art. For example, using F/M to represent pounds of five-day biochemical oxygen demand, BOD(5), per pound of microorganisms, A/F to represent pounds of oxygen per pound of BOD(5), and A/M to represent pounds of oxygen per pound of microorganisms, the following approximate relationships are known in the art:
______________________________________ Excess bio- logical solids Type (cells) of produced acti- per Typical vated Sludge lb. MLSS sludge age, BOD(5) content process days applied mg/l F/M A/F A/M ______________________________________ High 0.5- &gt;1 500-1000 1.0 0.7 0.7 rate 2 load Conven- &gt;2 1 &gt; 0.35 &gt;1000 0.3 1.0 0.3 tional &lt;6 &lt;3000 load Extend- &gt;6 &lt;0.35 &gt;0.2 &gt;3000 0.1 1.2 0.1 ed aera- &lt;20 &lt;5000 tion Low load extended aeration &gt;12 &lt;0.2 &gt;3000 0.05 1.5 0.08 (typical for oxi- dation ditch) ______________________________________
In order to remove nitrogen from a wastewater, in which it may be measured as total nitrogen or total Kjeldahl nitrogen, all systems using the wastewater as the chief organic carbon source for denitrification employ an alternating aerobic-anoxic sequence of stages, without intermediate clarification, to effect total nitrogen removal while attempting to avoid ammonia nitrogen bleedthrough. An oxidation ditch can be used for this purpose by controlling the level of aeration so that the mixed liquor is recirculated many times through alternating aerobic and anoxic zones prior to discharge from the channel of the ditch. To operate effectively, however, it is important that both zones be uninterrupted; i.e., aeration should occur at a single location immediately preceding the aerobic zone and should not recur until at least the end of the anoxic zone. If aeration occurs at only one location, so that there follows downstream thereafter one and only one aerobic zone, one and only one anoxic zone, and, if desired, an oxygen-deficient zone within the channel of the ditch, it is herein defined as point-source aeration. If there are multiple zones of each type, there is "multi-source aeration".
"Point-source aeration", "point-source mixing", and "point-source propulsion" are terms signifying that these three properties (hereinafter generally termed "point-source treatment") each originate at a single location within the channel of an oxidation ditch, in contrast to multiple locations therefor.
It is desirable that all of the mixed liquor of an oxidation ditch be homogeneously mixed with the inflowing waste, with the return sludge, and with an oxygen-containing gas which is hereinafter considered to be air. All three of these mixing operations can be simultaneously conducted, any two can be simultaneously conducted, or each can be separately conducted as either point source or multi-source operations.
When the mixed liquor is mixed with air, oxygen is dissolved in (i.e., transferred to) the mixed liquor. With respect to energy consumption, it is important whether such transfer is merely to a portion of the mixed liquor or to all of it. If the former, this portion must be aerated relatively intensively in order that after blending there will be the desired O.sub.2 content; it is herein termed heterogeneous aeration. If the latter, it is termed homogenous aeration which is herein specifically defined as the homogenous transfer of all required process oxygen into all of the mixed liquor of an oxidation ditch by direct-contact aeration. Either homogenous or heterogenous aeration can be point source or multi-source.
In all oxidation ditches of the prior art, the functions of aeration and propulsion of the mixed liquor are combined in a single device which is installed so that it contacts and mixes merely a portion of the mixed liquor with air. This device may be a horizontally shafted surface aerator, a vertically shafted surface aerator, or a single header of a directional mix jet aerator. A vertically shafted surface aerator may be high speed or low speed, and both horizontally and vertically shafted surface aerators may be fixed or floating. Such a device is hereinafter generally designated a pump/aerator.
Point-source propulsion signifies that all propulsive energy necessary for generating adequate velocity for all of the mixed liquor in the entire ditch is disposed at one location. The amount of this propulsive energy is roughly comparable to hydraulic head and can be measured as the length of channel between aerators. In the prior art, it is believed that directional mix jet aerators are capable of subsurface propulsion of the mixed liquor for 200--300 feet, that horizontally mounted surface aerators are capable of propelling the mixed liquor for 200-500 feet between pump/aerators, and that vertically mounted surface aerators can propel the mixed liquor at adequate velocities for up to 900 feet between pump/aerators when the concentration of mixed liquor suspended solids exceeds 3,000 mg/l or ppm.
It is a self-evident fact in the prior art that the pump/aerators are additionally limited not only as to the length of channel between pump/aerators but also as to volumetric capacity or volume of flow within the channel, commonly defined as circulation rate in cubic feet per second or cubic meters per hour. The result is that in a large oxidation ditch (which is typically of looped channel configuration) the pump/aerators must be installed at intervals along the channel to operate in series, creating multiple aerobic and anoxic zones. Because of the multiplicity of the zones, it is relatively difficult to control the respective volumes of the aerobic and anoxic zones.
Using the oxidation ditch 20 shown in FIG. 9 as a theoretical example of point source aeration and point-source propulsion, pump/aerator 21 divides its channel into intake channel 22 and discharge channel 23. Mixed liquor flows translationally in direction 30. Mixed liquor 24 is withdrawn to a clarifier which separates it into clarified liquor and settled sludge 25. Waste-water inflow 26 may be disposed within intake channel 22 but is preferably located upstream thereof within anoxic zone 28 which stretches from end 31 of aerobic zone 27 to its end 36. Aerobic zone 27 is considered to begin at pump/aerator 21.
Aerobic zone 27 can be operationally defined as beginning with the initial transfer of dissolved oxygen into the mixed liquor and as ending with the dissolved oxygen content (D.O.) dwindling to 0.5 mg/l at end 31. The length of aerobic zone 27 is determined by the input food supply, the concentration, mass, and type of microorganisms that are available, the D.O. content at the beginning of the zone, the K-rate or B.O.D. removal rate of the biomass, the O.sub.2 uptake rate of the biomass, the type of food (soluble and insoluble), the velocity of flow 30, and the temperature of the mixed liquor.
Anoxic zone 28 is characterized by having 0.0 to 0.5 mg/l of dissolved oxygen but is herein defined as the oxygen-depleted zone of activity for the heterotrophic facultative (denitrifying) bacteria and autotrophic (denitrifying) bacteria which obtain their needed oxygen from nitrate anions (liberating nitrogen as N.sub.2) and their food from organic carbon or H.sub.2 S. The organic carbon is available in: (1) the inflowing wastewater, (2) the cell biomass in the mixed liquor, or (3) the organic carbon adsorbed by the biomass of the mixed liquor. Theoretically 62.5 percent of the oxygen required for nitrification can be used for B.O.D. removal by denitrifiers, thus reducing power consumption for oxygenation.
As oxidation ditches are commonly designed for denitrifying at the present time, end 36 is apt to coincide with pump/aerator 21, and anoxic zone 28 can be volumetrically defined as the difference between the total channel volume and the volume of the aerobic zone. In such a commonly occurring situation, a downstream movement of end 31 to position 33 causes anoxic zone 27 to become shorter and smaller so that denitrification may become less complete, depending upon mixed liquor temperature and nitrate concentration in the mixed liquor at the beginning of the anoxic zone.
If, however, the ditch is large enough that anoxic end 36 is spaced from pump/aerator 21, movement of aerobic end 31 to position 33 causes anoxic end 36 to move upstream to position 37, and movement to position 33 also causes a downstream movement to position 38 without diminishing the volume of anoxic zone 28.
The volume between end 36 and pump/aerator 21 is herein defined as oxygen-deficient zone 29 which is characterized as having a D.O. of 0.0 mg/l (no measurable D.O. and no oxygen present in the form of nitrates) through which aerobic and facultative microorganisms circulate. Such an oxygen deficiency causes an oxygen-starved condition in the mixed liquor which is believed to create a "luxury" uptake rate of oxygen when initial contact of the microorganisms occurs with dissolved oxygen or even with undissolved air bubbles or undissolved oxygen. It is believed that this luxury rate occurs because the microorganisms adsorb oxygen with great avidity, immediately absorb the adsorbed oxygen to replenish their systems, and then promptly adsorb a further supply of oxygen in a normal manner.
The practical meaning of point-source treatment is that the volume of aerobic zone 27 can be controlled simply by varying the air or oxygen supplied to the mixed liquor by the point-source aeration device, thereby causing anoxic zone 28 merely to shift position if the oxidation ditch is long enough. Because the wastewater load to an oxidation ditch is typically subject to change on a daily, weekend, weekly, and/or seasonal basis, it is important to be able to control the respective lengths 34, 35, 49 of aerobic zone 27, anoxic zone 28, and oxygen deficient zone 29 in order to maximize BOD(5) removal and N.sub.2 removal by the nitrifiers and denitrifiers and thereby minimize the amount of oxygen that must be transferred by the point source aeration device. Point-source aeration and separately operated point-source propulsion greatly simplify such control.
In general, when an attempt is made to operate an oxidation ditch of the prior art with a single pump/aerator to aerate, mix, and propel the mixed liquor translationally through the channel of the ditch, the following problems, stated briefly, are typically encountered:
(1) a single pump/aerator cannot generate sufficient hydraulic head to pump the mixed liquor at an adequate circulation rate to produce and maintain a flow velocity that is high enough around the entire ditch to keep mixed liquor solids in suspension when MLSS concentration exceeds 3,000 mg/l and ditch length exceeds: (a) 900 feet and a vertically mounted surface aerator furnishes surface aeration, (b) 300-500 feet and a horizontally mounted rotor furnishes surface aeration and (c) 200 feet and diffusers or directional mix jet aerators furnish subsurface aeration.
(2) two or more pump/aerators cannot be concentrated (to operate as pump/aerators in parallel) in sufficiently close proximity for generating this necessary head at an adequate circulation rate and for transferring adequate oxygen at one point in an oxidation ditch to a mixed liquor containing more than 3,000 mg/l MLSS when the length of the endless channel exceeds 200-900 feet for specific aerators as previously set forth in (a)-(c) of (1);
(3) the dissolved-oxygen content of the mixed liquor cannot be changed without simultaneously changing its flow velocity since the channel circulation flow rate and velocity and O.sub.2 transfer rate are dependently related because they are imparted by the same device;
(4) an excessive energy price must be paid for heterogeneous aeration which is herein defined as intensively contacting, pumping, and aerating a portion of the mixed liquor and then blending the contacted-flow portion with the induced-flow portion, which is flowing past the aerator without receiving oxygen, to produce the desired average D.O. content in the mixed liquor;
(5) energy is wasted when prior art devices attempt to re-aerate freshly aerated mixed liquor that has been back-mixed into the intake of the aerator;
(6) When pumping and transferring oxygen to the mixed liquor by prior art aeration devices, it is not possible to compensate for depth variations beyond .+-.one foot (except jet aerators as shown in FIGS. 7 and 8 and diffusers combined with baffles) without using floating devices for the aerators; and
(7) aeration devices of the prior art are highly susceptible to icing and other cold weather problems (except jet aerators as shown in FIGS. 7 and 8 and diffusers combined with baffles), because surface aeration is employed.
These problems associated with prior art oxidation ditches are discussed in detail as follows with reference to FIGS. 10-16 of the drawings:
(1) Inadequate hydraulic head for entire ditch. This problem is created by the prior art attempting to conserve the momentum of the circulating mixed liquor by mounting the pump/aerator so that it interferes with the flow of this mixed liquor as little as possible and pumps a relatively small portion of the total flow. For example, the blades of one type of rotor dip into the surface of the mixed liquor to a depth of 4-12 inches, while the underlying liquor flows past undisturbed throughout a depth of 4-14 feet, minus the depth of blade penetration. After blending, as by means of an inclined baffle as seen in FIG. 1, its motion may be said to be "induced" by the energy in the contacted-flow portion. If hydraulic friction with the bottom and sides of the channel is sufficiently great downstream of the rotor, there is no available means to generate the necessary hydraulic head that will force the blended portions (i.e., the total flow) to move at a velocity sufficient to sustain its load of suspended solids until it returns to the rotor unless the oxidation ditch is a short one (e.g., 200-900 feet, depending on the type of aerator as previously set forth, when the MLSS concentration exceeds 3,000 mg/l, in (a)-(c) of (1).
(2) Inability to group pump/aerators in close proximity. If point-source treatment is to be maintained in a large oxidation ditch, it is necessary to increase horsepower input or to widen the channel and place two or more pump/aerator devices side by side or in otherwise close proximity in order to obtain adequate circulation rate. However, the devices of the prior art cannot be built to operate at more than 100 to 150 horsepower, depending upon the type of aerator, and are not adapted to operate in close proximity for pumping in parallel. Thus the only alternative is to lengthen the ditch, in order to obtain adequate ditch volume and the desired F/M ratio, and then to install a plurality of spaced-apart single pump/aerators furnishing multi-source aeration and multi-source propulsion.
(3) Simultaneous change of dissolved-oxygen content and flow velocity. The first three problems are closely interrelated and are discussed together in detail herein because any change in: (a) depth of submergence or speed of rotation for surface aerators or impellers or (b) liquid pumping rate or pressure of compressed air for jet aerators will simultaneously cause changes in both the D.O. content and the flow velocity of the mixed liquor. Such changes will be in proportion to the energy input to the pump/aerators. Nevertheless, variations in mixed liquor temperature or of inflowing wastewater characteristics, such as BOD(5) content or nitrogen content, may require, for example, an increase in D.O. content and a decrease in flow velocity.
In consequence, in a prior art oxidation ditch employing multi-source aeration from a plurality of spaced-apart pump/aerators, the lengths of the aerobic and anoxic zones may tend to vary according to inflowing wastewater and temperature conditions, but the fixed locations of the pump/aerators may inhibit flexible operation in accordance therewith. For example, as indicated in FIG. 10, conveniently spaced-apart rotor aerators 41 are often located, at least temporarily, within an anoxic zone 43 or are otherwise improperly spaced apart between aerobic zones 42 and anoxic zones 43 for proper nitrification/denitrification of the wastewater when utilizing organic carbon in the wastewater as the hydrogen acceptor.
Thus, rotor-equipped oxidation ditches have been operated for nitrification/denitrification by shutting down one or more rotors in order to obtain sufficiently long anoxic zones while hopefully retaining sufficient translational velocity to maintain the biomass in suspension. This procedure has been used, for example, at the Vienna-Blumenthal plant in Vienna, Austria, as discussed in "Process Design Manual for Nitrogen Control", October 1975, pages 5-42 through 5-45, which is available from the Office of Technology Transfer, U.S. Environmental Protection Agency, Cincinnati, Ohio 45268.
As stated on pages 5-48 of the EPA "Process Design Manual for Nitrogen Control":
"It has been found that the cage aerators which are typically employed in the oxidation ditch are not well suited to nitrogen removal applications. The cage aerator is not capable of simultaneously mixing and maintaining DO control; too much oxygen is imparted to allow development of alternating aerobic and anoxic zones while maintaining sufficient ditch velocities (one fps or 0.30 m/s) for prevention of settling of solids in the ditch. In one case, the problem was solved by providing separate submerged propellers for mixing which allowed the cage rotor to be managed for DO control alone." PA1 C=existing gas concentration PA1 t=time PA1 C.sub.s =saturation concentration of gas PA1 K=proportionality constant PA1 (a) the lack of collecting or gathering means for forcing all of the mixed liquor at a selected low D.O. content to flow past the aerator, and PA1 (b) the lack of a means for inhibiting back-mixing of freshly aerated liquor into the pump/aerator intakes. PA1 (1) dividing the mixed liquor in the channel of an oxidation ditch into an intake body and a discharge body, with the pump/aerator as the sole flow-through connecting means so that: PA1 (2) creating a differential head between the discharge body and the intake body on opposite sides of the barrier means and using it, particularly when the pump/aerator has no directional-flow discharge, for continuously moving the liquor in circuit flow through the channel from the discharge body to the intake body; PA1 (3) providing a mounting means for the pump/aerator so that all types of aerators can be mounted anywhere within the channel; PA1 (4) selectively providing controlled acceleration, uniform steady-state flow, and controlled deceleration for the liquor in the channel; and PA1 (5) providing a means for controlling the lengths of the aerobic and anoxic zones, independently of the flow velocity, while disposing all aeration apparatuses at a single location within the channel to obtain point-source aeration and point-source propulsion. PA1 (1) when the contents of the entire channel are quiescent and one pump/aerator is started, the other pump/aerator turns idly in reverse rotation; and PA1 (2) after about three minutes, the other pump/aerator slows, stops, and begins to turn idly in forward rotation.
In a large oxidation ditch employing the Carrousel system (slow speed, vertically shafted surface aerators), requiring over 100-150 horsepower for supplying the process O.sub.2 requirement, multi-source aeration is obtained by arranging the ditch to include several looped channels which are connected by channel turning points or bends. A vertical-shaft surface aerator 44 is installed in two or more of these bends, as indicated in FIG. 11. Nitrification and denitrification are controlled by turning off or cycling on and off one or more aerators, by varying the speed of selected aerators, or by varying the submergence of the aerators in order to control the translational flow and the lengths of the aerobic and anoxic zones 45, 46.
The directional-mix jet aerator system also appears to be propulsion limited because banks of headers 47 are typically spaced around an oxidation ditch at short intervals, as indicated in FIG. 12. The relatively high proportion of induced flow produces relatively short aerobic zones 48 and anoxic zones 49 as the mixed liquor moves counterclockwise through the channel. This system requires excessively high blower pressure to eject air at the bottom of a deep channel and also requires the operation and maintenance of a plurality of pumps to inject mixed liquor through the submerged jets for entraining the compressed air.
In the comparatively sized oxidation ditches of FIGS. 10-12, the spaced-apart aerators act as series pumps, but the velocity of mixed liquor flow does not change much in proportion to the number of units in operation. Referring to FIG. 10, the four 75-hp rotor aerators are each sized to supply about one-third of the total oxygen demand. Referring to FIG. 11, the three 100-hp low-speed surface aerators are each sized to supply about one-half of the total oxygen demand. Referring to FIG. 12, the six headers (each of which is designed to supply one fourth of the total oxygen demand) are individually connected to a 14-hp pump and are all connected to a 100-hp central compressor and one 100-hp standby compressor.
(4) Excessive energy consumption for heterogenous aeration. This problem is related to the propulsive limitation, the circulation rate limitation, and the aeration capacity limitation. The first three problems are also closely interrelated because the contacted-flow portion (actively pumped directly by the aeration device) not only receives all of the propulsive energy from the pump/aerator but also receives all of the dissolved oxygen by being contacted by or mixed with air, whereas the induced-flow portion directly receives neither propulsion energy nor oxygen. Therefore the final or maximum dissolved oxygen (D.O.) content of the contacted-flow portion must be great enough that the post-blended D.O. content of the total flow will be at the desired level, as indicated schematically in FIG. 13. The percentage of the total flow that is represented by the intensively aerated contacted-flow portion determines the maximum D.O. content to which this contacted-flow portion must be aerated in order to obtain a desired D.O. content in the blended flow.
The practical consequence of heterogeneous aeration is that, in general, when a pumped portion is intensively aerated and then blended with an induced-flow portion which is not aerated to produce a desired average dissolved-oxygen content, an energy price must be paid. The reason therefor is that when oxygenating water with air, the necessary driving force increases non-linearly as the dissolved-oxygen content of the water increases, as may be appreciated by considering the two-film theory of gas transfer.
This theory is based on a physical model in which two films exist at the gas-liquid interface. The two films, one liquid and one gas, provide the resistance to the passage of gas molecules between the bulk-liquid and the bulk-gaseous phases. For transfer of gas molecules from the gas phase to the liquid phase, slightly soluble gases encounter the primary resistance to transfer from the liquid film. FIG. 14 schematically illustrates the two-film gas transfer theory.
The rate of gas transfer, in general, is proportional to the difference between the existing gas concentration and the saturation concentration of the gas in solution. In equation form, this relationship can be expressed as: EQU dC/dt=K(C.sub.s -C)
where
K includes the effect of the resistance of either or both films and is also a function of the area of liquid-gas interface that exists per unit volume of fluid.
Oxygen is a slightly soluble gas in water so that traversing the liquid film from C.sub.1 to C in FIG. 14 is the main obstacle for the oxygen molecules. This situation may be thought of as a resistance to crowding by the oxygen molecules in the water; the more closely packed they become, the more strongly they resist the influx of additional molecules so that the change of concentration with time, dC/dt, decreases at a decreasing rate when a constant-volume system is subjected to a constant power input while mixing air with liquid.
Practical consequences of this phenomenon are illustrated in FIG. 15 which is a typical curve for oxygen uptake by water, using an upflow submerged turbine at constant power, without a draft tube, and with a compressed-air sparge beneath the deeply submerged turbine impeller (but with no upper impeller), within a large tank filled with deaerated tap water, dissolved oxygen concentrations being determined by the Winkler method and being corrected for cobalt ion content.
The decreasing slope of dissolved-oxygen concentration as a function of mixing and aerating time (at constant external power input) indicates that the water is increasingly resisting the attempted transfer of oxygen from the air bubbles. Thus if an intensively aerated portion, representing 33 percent of the channel flow by weight, contains 6.0 mg/l of D.O. and is blended with an induced-flow portion representing 67 percent of the channel flow by weight and containing 0.0 mg/l of D.O., the final blended dissolved-oxygen content is 2.0 mg/l at an average power input (measured in minutes) of 1.15 (corresponding to an average D.O. content of C=3.0 mg/l as the contacted-flow portion is aerated from 0.0 to a maximum of 6.0 mg/l) as compared to 0.4 (in minutes), if the entire blended contents of the channel were homogenously aerated to a final homogenous D.O. content of 2.0 mg/l (corresponding to an average D.O. content of C=1.0 mg/l as 100 percent of the channel flow is directly aerated from 0.0 to a maximum of 2.0 mg/l).
The manufacturer of the floating surface aerator shown in FIG. 3 has published a graph showing the cost (at an unknown date) for introducing 1,000 pounds of oxygen into water having various percentages of oxygen. The graph was developed to illustrate the benefits of adjusting horsepower input to the diurnal flow of municipal sewage as compared to the cost of using a steady horsepower input all during the day and the night. This graph is reproduced as FIG. 16.
Now consulting FIG. 16 for costs of oxygen transfer at average D.O. contents of 3.0 mg/l and 1.0 mg/l, $4.40 and $3.30 per 1,000 pounds of transferred oxygen are respectively obtained. The difference of $1.10 represents a 33 percent increase in power cost requirement for operation of a prior art oxidation ditch because of this practice of intensive aeration of a contacted-flow portion only and subsequent blending with an induced-flow portion. The smaller the portion of the total flow that is intensively aerated and the higher the final blended D.O. content that is desired downstream of a pump/aerator of the prior art, the higher the energy price that must be paid for such heterogenous aeration.
(5) Back-mixing. When blade and cage rotors as seen in FIGS. 1 and 3 and disc rotors as seen in FIG. 4 are operated, they recirculate on their surfaces or in their holes or throw backwards towards their intakes much freshly aerated liquor from which the microorganisms have not had time to absorb the dissolved oxygen. Thus this recirculated liquor has an O.sub.2 content that is characteristic of aerated water far along the curve of FIG. 15 where any additional input of oxygen meets increased resistance. The consequence is that energy is wasted by attempting to crowd in a supply of additional oxygen molecules. This practice is herein termed "back-mixing".
The manufacturer of the floating surface aerator shown in FIG. 3 was aware of this phenomenon and consequently provided the aerator with a variable-speed, horizontal-shaft rotor and an upstanding baffle and horizontally disposed splash pan behind the rotor "to prevent recirculation of freshly aerated fluid immediately back through" the rotor and in order to ensure that the rotor "is operating at all times at substantially its greatest efficiency, by receiving primarily that portion of the sewage liquid which has the lowest oxygen content".
As illustrated in FIGS. 5, 6, and 11, the Carrousel surface aerator hurls outwardly a large amount of liquor, air froth, and bubbles over the surface on its intake side, and this mixture is promptly drawn downwardly and swept toward the intake of the aerator. Such an intake would occur even if the aerator were equipped with a draft tube creating toroidal circulation. Indeed, the entire bend in the channel, within which such a vertically shafted surface aerator is mounted, is in a complete-mix state having relatively uniform D.O. content and consequently an aerator intake that pulls in a mixed liquor with a D.O. content not far below that desired as the product of the aeration zone (the hatched area in FIG. 5).
The directional-mix jet aerator system that is shown in FIG. 12 is also subject to back-mixing, for it has been observed to be capable of recirculating into its intake a portion of the mixture of air bubbles and freshly aerated liquor ascending from its jets.
A schematic analysis of prior art practice with respect to back-mixing, intensive aeration of a contacted-flow portion, and no aeration of an induced-flow portion, followed by blending of the portions to produce the desired D.O. content, is presented in FIG. 16. This situation is inherent in prior art oxidation ditches because of:
(6) Inability of fixed surface aerators to compensate for major depth variations. Fixedly mounted rotor and disc aerators are highly sensitive to depth variations of even a few inches and typically possess no means for elevating or lowering their relatively massive bulks by more than a foot, so that floating assemblies, such as the aerator seen in FIG. 3, must be used when flow equalization is desired. Vertical-shaft surface aerators have been plagued by mechanical stresses to the shafts and shock-load difficulties for the impellers because of variable submergence. Submergence of all surface aerators, including the floating types, is deliberately varied only for desired changes in O.sub.2 transfer, not for flow equalization or for velocity control. The oxidation ditches equipped with the floating horizontal-shaft rotors seen in FIG. 3, however, may be built with a cross-sectional configuration permitting considerable flow equalization.
(7) Inability of surface aerators to operate during adverse weather conditions. All types of surface aerators, both fixed and floating, which have been installed in oxidation ditches have commonly been afflicted with aerosol spray and with severe icing on their surfaces during freezing weather. Only the directional-mix jet aerator is immune to weather conditions. Although the submerged turbine aerator (which is widely employed in aeration lagoons and complete-mix basins), is not affected by adverse weather conditions, it has not been used in oxidation ditches.
In summary, prior art oxidation ditches provide zones for nitrification and denitrification but are so limited by propulsion capability and/or circulation rate and/or aeration capacity that their aeration devices must be disposed sequentially (in series) throughout the ditch and be spaced at distances apart that are based upon propulsion capabilities and/or circulation rates and/or aeration capacities without regard to lengths of the nitrification and denitrification zones. In other words, they are characterized by inherent rigidity and are incapable of adjusting to wide variations in flow rates and temperatures.
One reason therefor is that in prior art oxidation ditches the same aeration apparatuses supply the dissolved oxygen and create both induced flow and contacted flow of the mixed liquor so that elapsed times for circuit flow, oxygen supply, and aerobic/anoxic volume fractions are interrelated. For example, if the D.O. in a prior art oxidation ditch is too high, the aerobic zone is too long. If the oxygen supply is cut back in order to correct this situation, the flow velocity is simultaneously reduced. Yet a minimum flow velocity is required in order to maintain the bio-mass in suspension.
All of these enumerated problems have been satisfactorily solved by the inventions disclosed in the parent applications, Ser. No. 649,995 and Ser. No. 848,705, which provide a barrier means in sealed combination with a pump/aerator for:
(a) flow of mixed liquor occurs only once each circuit flow through the pump/aerator, PA2 (b) back-mixing from the exit to the entrance of the pump/aerator is completely prevented, and PA2 (c) none of the flow in the channel is induced flow;
Essentially, the inventions disclosed in the parent applications solve these problems by providing point-source aeration and mixing with a means for gathering ALL of the mixed liquor, and thereby all of the floc particles, into at least one treatment center or passageway wherein or at the entrance or exit of which at least one pump means and at least one aeration means are disposed, whereby homogeneous aeration occurs, the pump means and aeration means being independently operable when an axial-flow pump is used for propulsion and a separate device is used for aeration (e.g., diffusers, jet aerators, and surface aerators). These inventions are generally referred to hereinafter as a barrier oxidation ditch.
The barrier, however, is useful for selective acceleration and deceleration, for providing homogeneous aeration, and for imparting an adequate hydraulic head to a desired volume of mixed liquor to propel it through distances unknown to the prior art, but it appears to be a hindrance with respect to conservation of momentum in the steady-state translational flow of the mixed liquor. As a demonstration of this concept, a barrier oxidation ditch having two impeller-type aerators, as described in Example 1 of parent application Ser. No. 848,705, has been observed to function as follows:
A means for increasing flow efficiency of a directional-discharge barrier oxidation ditch by conserving its translational-flow momentum, while retaining accelerative and decelerative advantages of the barrier, is accordingly needed.
When an existing oxidation ditch of the looped-channel type, containing a plurality of spaced-apart aerators, is converted to a barrier oxidation ditch having point-source aeration, it is possible under certain conditions of food supply, F/M ratio, and temperature to have oxidation ditch dimensions so restricted and total oxygen demand so great that the largest aeration device is inadequate and two or more suitably sized aeration devices cannot be installed in combination at one location. It therefore becomes necessary to have aeration devices located at more than one location in the channel, i.e., to have multi-source aeration. However, both point-source and multi-source aeration can be either homogeneous or heterogeneous. Homogeneous aeration, mixing, and propulsion simply require the use of a barrier as a gathering or collecting means to control the flow of the mixed liquor by forcing all of the mixed liquor to enter the discharge passage.
Nevertheless, it may be undesirable to have two barriers providing homogeneous mixing or aeration because it may be more desirable to conserve momentum. In other words, homogeneous aeration is more efficient in terms of energy consumption for mixing an oxygen-containing gas with the mixed liquor, but when two or more barriers are needed, it may be more desirable to have a selected degree of heterogeneous aeration, coupled with conservation of a selected amount of momentum, than to have homogeneous aeration at both barriers. Further, if homgeneous mixing is critical because of very high NO.sub.3 --N in the inflowing wastewater and because of a need for maximizing use of the available anoxic volume, it may be desirable to utilize a barrier to obtain homogeneous mixing of mixed liquor with raw wastewater and return sludge while compromising as to heterogeneous aeration in order to have conservation of momentum. A discharge passage is herein defined as a flow channel of smaller cross section than the endless channel of the oxidation ditch and through which the mixed liquor moves past a barrier disposed across the endless channel. A discharge passage includes a discharge duct, a discharge slot, a vertically disposed draft tube, and the like.
A barriered pump assembly is herein defined as any combination of: (1) a pump means for forcing mixed liquor to move through a discharge passage from an intake channel to a discharge channel and (2) a barrier means for forcing up to all of the mixed liquor to enter the discharge passage. If an aeration means is additionally provided, the term used herein is a barriered pump/aerator. If streams of raw wastewater and/or return sludge are connected to the discharge passage and no aeration is performed, the term used herein is a barriered pump/circulator.
The barrier means comprises: (1) at one extreme, a water barrier in combination with an extended intake baffle which is connected to the discharge passage, (2) at the other extreme, a complete barrier which sealably separates the intake channel from the discharge channel, being connected thereto solely by the discharge passage; and (3) intermediate therebetween, for example: (a) an adjustably apertured barrier assembly and (b) a combination of pump/aerators, connected in side-by-side relationship to form a barrier, and a jet-pump aerator which is disposed in flow connection with a flap-controlled opening through the barrier. However, a thin wall member which is disposed across the endless channel but which permits a relatively large amount of induced flow to pass by is herein termed a baffle.